Ultra-fine and stable refractory particles, if properly distributed within a metal matrix, impart excellent microstructural stability to the matrix even at temperatures up to as high as 0.9 of the absolute melting point (T.sub.m) of the metal. As used herein, the term ultra-fine particle shall be deemed to mean a particle, the volume of which approximates the volume of a sphere having a diameter which is less than 0.1 micron. These materials, comprising a metal matrix and an ultra fine dispersion of high thermal stability particles, as exemplified by the refractory ceramics, are referred to in the art as dispersion-strengthened (DS) materials. Such materials have excellent strength retention capability at and after elevated temperature exposures. In spite of their unique characteristics (i.e., strength and stability at high temperatures), the effective commercialization of such DS materials has been slow, mainly due to the high processing cost associated with the manufacturing of useful DS materials.
In dispersion strengthened copper, for example, the majority of the DS copper materials utilize a refractory oxide as the dispersoid (sometimes referred to as ODS copper). Various techniques have been developed to process oxide dispersed copper. Most of these techniques utilize the copper and oxide materials in a powdered form as the starting materials, and they differ mainly in the method by which the oxide powder particles are introduced into the copper powder matrix. Among the various processing methods currently available, those which provide ODS copper by the use of an internal oxidation (IO) processing technique seem to have gained the most popularity. It has been demonstrated that IO ODS copper has superior mechanical properties over oxide DS copper materials manufactured by other known processing methods. Such superior properties, however, are achieved at a penalty inasmuch as making ODS copper using an IO technique is a very tedious and time consuming process, which factors contribute to the very high processing costs thereof. Consequently, industrial applications of ODS copper have not been very wide spread.
While, for purposes of clarity, concepts relating to DS metals are generally discussed herein using copper (Cu) as an example of the metal matrix material, the processes and materials discussed herein are applicable to other types of metal matrices, such as aluminum (Al), iron (Fe), and nickel (Ni), for example.
In recent years, several other methods for making DS materials have been developed . U.S. Pat. No. 4,647,304 discloses a method for mechanically forming dispersion strengthened metal powders by the use of a milling process in the presence of cryogenic materials. European Patent No. 0180144 shows a method for strengthening an aluminuim-lithium-magnesuim (Al-Li-Mg) material through the mechanical alloying of Al with carbides, oxides and silicides. European Patent No. 0184604 shows yet another method by which oxides can be formed inside a metal matrix wherein the matrix material, formed as a porous powdered-solid material with O.sub.2, is placed in a high pressure casting mold together with a second molten metal. The presence of the second metal in a molten state in contact with the powdered solid leads to a chemical reaction that promotes the formation of oxides inside the matrix. All of these methods are costly because of the many processing steps involved.
U.S Pat. Nos. 4,436,559 and 4,436,560 disclose a method for the manufacture of copper base materials dispersed with boride particles. The material is intended to be electrically conductive for use in providing electrical contacts, for example, where high resistance to adhesion, wear and arcing are desired. In these patents, the size of the boride particles range from 0.1 micron to as high as 20 microns and the presence of such a large proportion of particles having sizes substantially greater than 0.1 micron does not produce an adequate dispersion strengthening effect. In addition, the boride particles as disclosed are located substantially only at, or very near, the surface portion of the copper matrix, preferably within a depth of only 0.01mm. to 1.0mm. from the surface. Such a material will not have any useful bulk strengthening properties obtained from the boride dispersion.
U.S. Pat. No. 4,440,572 discloses a method for producing alloy modified ODS copper materials, the ODS copper alloys set forth therein using only refractory oxide particles, e.g. aluminum oxide, as the dispersoid.
While other recent patents have disclosed methods for incorporating borides into non-copper matrices, they generally use relatively large size boride particles. For example, U.S. Pat. No. 4,678,510 shows a method of compacting and sintering of powders with carbon, copper and nickel boride. The particles obtained through this process have dimensions greater than 1.0 micron.
U.S. Pat. No. 4,673,550 discloses a method for preparing, milling and mixing powders that can react during the mixing to form borides. The process focuses on making other composite materials rather than making DS materials.
U.S. Pat. No. 4,677,264, shows how an electrical contact material can be manufactured using an atmospheric sintering and pressurized sintering of powders and a subsequent infiltration thereof. Through this process, pre-manufactured boride powders of about 40 microns in size are used. U.S. Pat. No. 4,693,989 discloses a method for preparing and sintering refractory metal borides of high purity, but does not deal with techniques for making DS materials.
U.S. Pat. No. 4,690,796 discloses a process for producing aluminum-titanium diboride composites, the process therein involving entraining agglomerated particles in a carrier gas that passes through a hot zone (plasma) and then resolidifying the high temperature treated particles by cooling, using an rapid solidification process (RSP) technique. The resultant material contains particles of TiB.sub.2 which are generally less than 20 microns in size, but are no less than 6 microns.
All of the above processes involve numerous and costly steps which do not lead in general to the manufacture of an ultra-fine dispersoid within a DS material. It is desirable to develop better techniques which are much less tedious and time consuming and less costly, by which an ultra-fine refractory boride dispersoid can be incorporated into a metal matrix. In addition, it would be greatly advantageous if the microstructure and composition of the material produced by such a method can be tailored so as to enhance the properties required for many specific engineering applications. Moreover, such a process and the materials produced would be of considerable technological and commercial value if such materials can utilize not only a copper matrix, but also aluminum, iron and nickel matrices as well.